Avoid splitting reaction SMILES at dative bonds

[avaucher]

As indicated in rxn4chemistry/rxnmapper#50, splitting reaction SMILES at ">" characters is not always safe anymore after RDKit introduced dative bonds with the "->" notation. This PR fixes this.

[cla-bot]

Thank you for your pull request and welcome to our community. We could not parse the GitHub identity of the following contributors: Alain Vaucher.
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[avaucher]

@cla-bot check

[cla-bot]

The cla-bot has been summoned, and re-checked this pull request!

[drugilsberg]

LGTM